The subject of the present invention is a composition that can be used as a blowing agent, a polymeric composition comprising it, and a process for producing an expanded polymer by extrusion using this same composition. It relates more particularly to a process for producing extruded expanded polystyrene panels.
In general, expanded polymers are manufactured by melting granules of a thermoplastic polymer under pressure in an extruder and then by blending a volatile blowing agent into the molten resin under controlled temperature and pressure conditions allowing this blend to be maintained in an essentially liquid or viscous state. Next, this blend is extruded through a die in a zone at a lower pressure and at a fixed temperature (generally at atmospheric pressure and at room temperature) so as to obtain a solid polymeric structure of the desired shape, which has a multiplicity of substantially closed cells, the size of which is generally between 0.1 and 1.5 mm. The term xe2x80x9cclosed cellsxe2x80x9d is understood to mean cells which are not interconnected, in other words, cells which are separated from one another by a wall of polymeric material whose thickness is of the order of one micrometer.
Such a polymeric structure can be used in the form of sheets for use as thermoformable material for the production of food packaging, or else in the form of panels suitable for thermal insulation in the building field. The thickness of the expanded sheets formed by extension is generally between 0.5 and 10 mm, preferably between 1 and 8 mm; the thickness of the insulation panels is usually between 10 and 200 mm, preferably between 20 and 160 mm. The density of an expanded material formed by extension is often between 10 and 250 kg/m3, that of the sheets preferably being between 50 and 150 kg/m3, while that of the panels is between 20 and 60 kg/m3, even more preferably between 25 and 50 kg/m3.
The thermal insulation properties of expanded polymers derive to a large extent from the thermal conductivity of the gas which fills the closed cells of the material, which gas is introduced during the manufacturing process as a blowing agent.
1,1-Difluoro-1-chloroethane, of formula CF2Clxe2x80x94CH3 (usually called HCFC-142b), and its mixtures with difluorochloromethane (usually called HCFC-22) are the blowing agents most widely used in the world for manufacturing expanded polystyrene panels intended for thermal insulation. However, during ageing of the expanded polymers these gases tend to migrate through the walls of the closed cells of the insulating material, owing to their permeability in this medium, and join the atmosphere where, because of their non-zero ODP (Ozone Depleting Potential) they are liable to represent a threat to the environment, especially in the case of the stratospheric ozone layer.
It is therefore necessary to substitute HCFCs (hydrochlorofluorocarbons) currently used in extrusion-expanded polymers with HFCs (hydrofluorocarbons) whose molecule, not containing a chlorine atom, has an ODP of 0. Moreover, HFCs have been developed for the refrigeration field. The need to make such a substitution is made all the greater by a forthcoming legal prohibition of HCFCs in Europe.
International Application WO 98/03581 describes the use of HFC-134 (1,1,2,2-tetrafluoroethane) for the production of thermoplastic foams. However, although HFC-134 advantageously has an ODP of 0, it is not a product widely available from the industrial standpoint.
The United Nations, within the context of their UNEP (United Nations Environment Programme), published a report in April 2000 entitled xe2x80x9cReport of the Technology and Economic Assessment Panelxe2x80x9d dealing with the Montreal Protocol relating to substances which deplete the ozone layer. This report (on pages 62 and 63) recommends HFC-152a (1,1-difluoroethane) as an HFC-type blowing agent intended for the manufacture of expanded polystyrene panels. However, this compound has a high permeability in polystyrene which, as a result of the gradual substitution in the closed cells of the blowing agent with air, which has a higher thermal conductivity, results in a rapid deterioration over time of the insulating properties of the corresponding polystyrene panels.
Among the HFC substitutes which may be envisaged, HFC-134a (1,1,1,2-tetrafluoroethane) may, because of its low thermal conductivity, make it possible to obtain insulating properties for the expanded polymer formed which are equivalent to those obtained with HCFC-142b. In addition, both these molecules have a low permeability through the polymer so that, advantageously, the good thermal insulation properties are maintained over time, even after many years. Furthermore, HFC-134a is very widely used on an industrial scale because of its applications in the refrigeration field.
However, the abovementioned UNEP report teaches that it is technically difficult to manufacture expanded polystyrene by extrusion using HFC-134a as the only blowing agent, because of the very low solubility of this compound in molten polystyrene.
This is because, in order to obtain an expanded polymer of high quality, the solubility of the blowing agent in the molten polymer must be sufficiently high, especially under the temperature conditions in which the blowing agent/molten polymer blend remains in the extruder chamber and passes through the die. The corresponding temperature range is generally between 100 and 300xc2x0 C., preferably between 110 and 200xc2x0 C. If this solubility is insufficient or too low, the blend lacks homogeneity and the blowing agent vaporizes prematurely at the die exit, so that a high proportion of the cells of the material have their walls ill-formed or absent. This results in a reduction in the stiffness of the material and, more generally in its mechanical properties. In such a case, too high a density for the desired application is also observed.
To solve this problem, the UNEP report suggests the use of HFC-134a in combination with an organic blowing co-agent, without however recommending a specific organic compound.
It is therefore an object of the present invention in particular to provide a blowing agent for the production of insulating thermoplastic foam which does not represent a danger to the stratospheric ozone layer (ODP equal to 0), based on industrially available HFC, and being able to substitute for HCFC-142b because of substantially equivalent thermal conductivity, permeability and solubility in the molten polymer.
It has now been found that cyclopentane surprisingly has, unlike other similar organic compounds, a particularly high solubility in molten polystyrene and that mixtures of cyclopentane with HFC-134a are well suited to the production of expanded polymers by extrusion, and especially of expanded polystyrene panels.
The subject of the invention is therefore firstly a composition which can be used as a polymer blowing agent, comprising:
from 60 to 98%, preferably from 70 to 96%, of HFC-134a, and
from 2 to 40%, preferably from 4 to 30%, of cyclopentane.
The percentages given in the present text are, unless otherwise indicated, percentages by weight. This composition can be directly prepared by simply blending the constituents and is injected into the extrusion chamber at a pressure greater than 10 MPa. It can be also obtained by separate injection of each constituent in the extrusion chamber. It may furthermore include from 0.5% to 33%, preferably from 1% to 26%, of one or more other blowing agents such as carbon dioxide, a hydrocarbon, an alcohol such as ethanol, an ether such as dimethyl ether, ethyl chloride or even another HFC.
The composition according to the invention, because of its solubility in molten polymers, and especially in polystyrene, similar to those of HCFC-142b and HFC-152a, makes it possible to obtain a homogeneous expanded polymer having a uniform distribution of closed cells in the material, and, consequently, the required mechanical properties.
The invention also relates to a polymeric composition which can be used in the production of an expanded polymer, comprising:
from 5 to 15%, preferably from 7 to 12%, of a blowing agent as defined above; and
from 85 to 95%, preferably from 88 to 93%, of a thermoplastic polymer.
This composition is produced by hot-blending its ingredients in the extrusion chamber at a temperature between 100 and 300xc2x0 C., preferably between 110 and 200xc2x0 C. and at a pressure between 5 and 30 MPa. The pressure in the zone immediately upstream of the die is usually between 5 and 20 MPa, preferably between 6 and 15 MPa. The latter pressure range is particularly advantageous as regards the economics of the process and the possibility of using an existing extrusion device, such as that used with HCFC-142b. Using a pressure within the latter range advantageously results in high solubility of the blowing agent according to the invention in the thermoplastic polymer.
The blowing agent according to the invention may be used in combination with any thermoplastic resin suitable for extrusion, such as polyethylene, polypropylene, polystyrene, and polyvinyl chloride.
According to a preferred variant, the thermoplastic polymer is polystyrene.
Auxiliary compounds may also be included in the composition, such as one or more fire retardants, nucleating agents intended to improve the homogeneity of the cellular structure, colouring agents and plasticizers, such as polyarylalkanes (xylylxylene, benzyltoluene or dibenzyltoluene). The amount of these components is generally between 0.01% and 10%, preferably between 1% and 5%.
Finally, the invention relates to a process for manufacturing an expanded polymer comprising the extrusion through a die of a polymeric composition as defined above.
According to a preferred variant, the extrusion is carried out in an extruder suitable for the use of HCFC-142b as blowing agent. This possibility results from the high solubility of an HFC-134a/cyclopentane composition as described above and is advantageous as it does not require new equipment to be developed.
This process is preferably carried out for the manufacture of expanded polystyrene panels suitable for thermal insulation, especially for buildings.